Subject: Re: [Isis-wg] checksum calculation doubt

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Fri, 25 Jan 2002 23:43:59 -0500 (EST)

Yeah, I think Mike owes you dinner, John. I may have typed the words in, but
it was Tony that got excited about that sort of stuff, and it was his
idea. It seemed harmless to put it in. Obviously it
doesn't prevent malicious errors. I suspect that in all the years of
running IS-IS it didn't catch any accidental errors, either, but who knows.

Was the quoted text correct, by the way?  "treating the source ID
field as the first octet" Wouldn't it be the first 6 octets (oh yeah,
the ID-length number of octets)?

And without looking at the packet format, I assume that the first
thing in the packet must be the source ID? Otherwise you'd get a different
checksum if you just ignore that text and do the checksum from the beginning
and initialize to 0. If the first thing is the source ID (it must be),
then starting after the source ID, with the value that the checksum would
be at that point, and starting at the beginning with checksums initialized
to 0 would be the same answer, and as Mike pointed out, you'd never
be able to detect from the outside of the box which strategy the router
was doing.

I could claim I was saving computation because the checksum could be
precomputed for the first 6 bytes. :-)

Radia

>>John Harper said:

>>	Nah, I bet you dinner it was Tony. This is exactly the kind of thing he 
got 
>>	excited
>>	about.

	At 18:06 25/01/2002 +0000, mike shand wrote:
	
	>But perhaps Radia has stronger opinions on this one, since I think it 
was 
	>her idea.
	
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From: mike shand <[email-masked]>
Date: Mon, 28 Jan 2002 09:58:02 +0000

>running IS-IS it didn't catch any accidental errors, either, but who knows.

My apologies Radia. I should have guessed that this had Tony's hallmark:-)

For those reading this list who may otherwise be confused, we are not of 
course referring to either of our illustrious WG chairs, but to Tony Lauck 
who ran the network architecture group in DEC when Radia, John and I worked 
there sometime last century.

         Mike